1. Field of the Invention
The invention relates to a vacuum cleaner including a housing, a suction unit accommodated in the housing and drivable by means of a first electric motor, a suction attachment having a rotatable brush which is drivable by means of a second electric motor, and an electrical control unit for controlling an electric current through the first motor.
2. Description of the Related Art
A vacuum cleaner of the type defined in the opening paragraph is known from EP-B-0 458 057. The known vacuum cleaner has a pressure sensor for measuring an air pressure in the suction attachment. When during operation a user of the vacuum cleaner moves the suction attachment over a surface to be cleaned, the air pressure in the suction attachment fluctuates because the air drawn in by the suction unit via the surface and the suction attachment encounters a fluctuating resistance owing to irregularities in the surface. The pressure fluctuations in the suction attachment have an amplitude which depends on the type of surface over which the suction attachment is moved. The pressure fluctuations are comparatively large when the surface is for example a carpet, whereas the pressure fluctuations are comparatively small when the surface is smooth, as for example in the case of wooden floors or tile floors. The control unit of the known vacuum cleaner comprises a microprocessor which detects the type of surface on the basis of the amplitude of the pressure fluctuations measured by means of the pressure sensor in the suction attachment. The microprocessor controls the currents through both the first motor, which drives the suction unit, and the second motor, which drives the brush, depending on the detected type of surface. In this way, the control unit adapts the suction power of the suction unit and the speed of rotation of the brush to the detected type of surface, so that an optimum relationship between cleaning performance, ease of use, current consumption and noise production of the vacuum cleaner is obtained for different types of surface.
A disadvantage of the known vacuum cleaner is that the pressure fluctuations in the suction attachment are influenced by the manner in which the user handles the vacuum cleaner. If the user, for example, holds the suction attachment in a fixed position on a carpet or moves the suction attachment with a comparatively large hold-down pressure over a carpet, the pressure fluctuations in the suction attachment have a comparatively small amplitude, as a result of which it is possible that the control unit detects a smooth type of surface and does not adapt the suction power of the suction unit and the speed of rotation of the brush to the carpet in an optimum manner. If the user moves the suction attachment away from a smooth floor and puts it back several times in succession, the pressure fluctuations in the suction attachment have a comparatively large amplitude, as a result of which the control unit may detect a carpet. Such misinterpretations by the control unit of the known vacuum cleaner can be prevented by providing the microprocessor of the control unit with a number of additional control rules. However, as a result of this, the control unit becomes comparatively complicated and slow.